The present invention relates to extruded honeycomb structures and more particularly to apparatus for extruding honeycombs with improved dimensional accuracy.
The manufacture of inorganic honeycomb structures from plasticized powder batches comprising inorganic powders dispersed in appropriate binders is well known. U.S. Pat. Nos. 3,790,654, 3,885,977, and 3,905,743 show the production of ceramic products using these manufacturing processes, while U.S. Pat. Nos. 4,992,233 and 5,011,529 describe honeycombs of similar cellular structure extruded from batches incorporating metal powders.
A variety of different honeycomb configurations can be formed by extrusion, including curved honeycombs such as disclosed in U.S. Pat. No. 5,456,965. These variations and other geometric properties of the extruded honeycombs can be developed through techniques such as varying the rate of delivery of extrudable material through the die forming the honeycomb. However, the vast majority of worldwide ceramic honeycomb production, which currently serves a large market for honeycomb-supported catalysts used in the abatement of combustion pollution from motor vehicles and stationary combustion sources, is concentrated on the efficient manufacture of honeycombs of right cylindrical configuration. These shapes, which may be of circular, oval, or other closed cylindrical cross-section, are designed to provide straight-through gas-flow channels running exactly parallel to the axis of extrusion.
Ceramic honeycomb substrates for automotive applications are generally produced by cutting and firing individual pieces from a stream of honeycomb extrudate, or by cutting the pieces from a dried green or fired ceramic "log" of extrudate which may be of meter or greater length. To meet customer requirements for the subsequent catalyst coating and "canning" of these substrates in suitable metal enclosures, it is important that the logs and pieces cut from the logs have sides which are absolutely straight and parallel.
The production of a straight stream of extruded material is quite difficult; in most cases at least some "bowing" of the extrudate, attributable to uneven flow of material through the extrusion die, is observed. This bowing can be caused by non-uniform flow characteristics in the batch, but more commonly is due to uneven flow resistance across the face of the extrusion die. Even with careful attention to die fabrication, uneven machining resulting from facts such as progressive tool wear, misalignment of feed holes and discharge slots, and non-uniform exposure to chemical machining and/or plating electrolytes often result in at least some bowing tendency being "built in" to most honeycomb extrusion dies during manufacture.
One prior art approach to the resolution of this problem involves the use of a device called a "bow deflector". This deflector comprises a drilled or otherwise perforated "breaker" or aperture plate installed immediately upstream of the die with respect to the flow direction of the feed stream which has a varying or tapered thickness.
A typical example of a conventional bow deflector of this type is schematically illustrated in FIG. 1 of the drawing. FIG. 1 shows a tapered aperture plate 2 disposed upstream of an extrusion die 4 so that a plasticized powder batch material flowing toward the die in the direction of arrow 6 must traverse apertures 8 in the plate (bow deflector) before reaching the die. The taper in deflector 2 is introduced so that batch material traversing the thickest section of the plate will traverse the longest apertures (8a), and so will experience more flow resistance (a higher pressure drop) and produce less volumetric flow into and through the extrusion die for a given pressure than material traversing the shortest apertures (8b) in the thinnest section of the plate. Apertures in between these two extremes will have an intermediate effect on batch flow to the die.
The end effect of inserting this plate in front of the extrusion die is that the pressure and feed rate of extrudable material to each portion of the die will be inversely proportional to aperture length in the bow deflector behind that portion. This produces a flow velocity gradient across the diameter of the bow deflector in the direction of maximum taper. Given proper alignment of the bow deflector with respect to the die, the flow gradient from the deflector can theoretically counterbalance a pre-existing flow gradient from the die, resulting in an extruded log with much less bend or bow.
While this approach is sound in theory, two problems have been identified in practice. The first problem concerns the direction of the bow and the second concerns the severity of the bow. The taper direction of any bow deflector must of course be aligned to counteract the direction of bow induced by the die. Thus provision must be made for rotating the bow deflector plate behind the die, in order to obtain cancellation of the bowing effects produced by each.
With respect to the severity of bow, the difficulty is that the amount of bow correction available from a given deflector plate is a single, relatively fixed value directly related to the degree of taper of that plate. Within reasonable limits, almost any amount of bow is correctable given the right taper, but experience teaches that the amount of taper angle required to correct a given degree of bow will vary significantly from die to die, and will also vary somewhat depending on the age of the die and of the bow deflector itself. Thus each manufacturing facility must maintain a relatively large and expensive inventory of bow deflectors to meet any extrusion condition which might eventually be encountered in production.